1. Field of the Invention
The present invention relates to a technique for judging an occurrence of double feeding in which a plurality of sheets are fed together with the sheets overlapping one another.
2. Description of the Related Art
A sheet feeding apparatus such as a recording apparatus, an image reading apparatus, and a facsimile machine includes a feeding mechanism for feeding a sheet such as a document and a recording sheet. This sheet feeding apparatus including the feeding mechanism needs to prevent what is called double feeding (overlapping feeding) in which a plurality of sheets are fed together with the sheets partly or completely overlapping one another. As one example for preventing the double feeding, there is conventionally known a sheet feeding apparatus using an ultrasonic sensor. The ultrasonic sensor includes an ultrasonic generator and an ultrasonic receiver disposed opposite to each other with a sheet feeding path interposed therebetween. An amount of attenuation of an ultrasonic wave in the sheet is different between feeding of a single sheet between the ultrasonic generator and the ultrasonic receiver and feeding of a plurality of sheets therebetween. Thus, this sheet feeding apparatus compares the amount of the attenuation with a predetermined threshold value to judge whether the double feeding has occurred or not.